Gas cushion control for seismic gas exploder

ABSTRACT

Disclosed is a seismic gas exploder in which the exhaust system is controlled to cause the retention of exhaust gases in a pressurized volume within the body of the exploder. This volume of gas then acts as an energy absorbing cushion or shock absorber against which the gas exploder can bounce and rebound without injecting unwanted secondary shock energy into the ground. Retention of the exhaust gases in this manner allows near peak pressure to be developed from a charge.

United States Patent [191 Kelly et al.

1 Feb. 6, 1973 GAS CUSHION CONTROL FOR SEISMIC GAS EXPLODER [73]Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Oct. 5, 1970 [21] App]. No.: 77,767

[52] US. Cl. ..18l/.5 NC, 181/.5 EC [51] Int. Cl. ..G0lv H10 [58] Fieldof Search ..181/.5 NC, .5 EC; 137/469; 102/25 [56] References CitedUNITED STATES PATENTS 3,588,801 6/1971 Leonard ..l81/.5 NC 3,198,2828/1965 Dunaway .....l8l/.5 NC

3,480,102 11/1969 Kilmer ..l8l/.5 NC

' 3,422,447 l/l969 Kilmer t t ..l8l/.5 NC 3,052,259 9/1962 Velden..l37/469 I ii I 1| 1 3,055,388 9/1962 Tebb et al. ..l37/469 FOREIGNPATENTS OR APPLICATIONS 808,964 3/1969 Canada "181/15 NC PrimaryExaminer-Benjamin A. Borchelt Assistant Examiner-N. MoskowitzAtI0rneyJames 0. Dixon, Andrew M. Hassell, Rene E. Grossman, HaroldLevine and Melvin Sharp [57] ABSTRACT 17 Claims, 7 Drawing FiguresPmiminrwsma I 3.715.007

GAS CUSHION CONTROL FOR SEISMIC GAS EXPLODER 'IHis invention relates topetroleum exploration, and more specifically to the generation ofseismic waves from which seismic recordings are made. Still morespecifically the invention relates to a gas exploder used to generateseismic waves.

In the petroleum exploration industry a device called a gas exploder, orcommonly a gas gun, is frequently used to generate seismic waves. Atypical gas gun comprises a movable pan-shaped member affixed to a heavystationary member, forming a combustion chamber between the two members.The stationary member is normally attached to a weighted object such asa truck. In use, a mixture of oxygen and propane is fed into thecombustion chamber and then exploded by means of an ignition spark. Thisexplosion drives the pan-shaped member into engagement with the earthwith sufficient force to generate a detectable seismic wave. The forceof the explosion will also raise the stationary member with respect tothe pan-shaped member.

The exhaust system in a conventional gas gun comprises a movable pistonmounted in a cylinder located in the stationary member. Tension in aspring forces the piston downward in the cylinder such that the pistonoriginally occupies the entire volume of the cylinder. The explosionforces the piston upward in the cylinder. The tension in the spring ismatched to the desired explosive force such that when the piston hasraised to a specific height the exhaust gases and pressure escapethrough exhaust ports in the cylinder wall. This sudden release ofpressure from the combustion chamber results in the stationary memberrebounding against the pan-shaped member and introducing secondaryseismic waves into the earth. These secondary waves interfere withseismic recordings being made from the original seismic wave. Also,maximum fuel efficiency cannot be obtained because peak pressure is notachieved in the combustion chamber.

Accordingly, it is an object of the present invention to provide a gasgun wherein creation of secondary shock waves is avoided.

It is a further object of the invention to provide a gas gun whereincreation of secondary shock waves is avoided by means of controlling theexhaust system of the gas gun.

Another object of the invention is to provide a gas gun wherein'peakpressure and maximum efficiency are achieved from a charge.

Another problem with the conventional gas gun is failure to obtaineffective purging of the combustion chamber after ignition of a charge.If the tension in the exhaust piston spring is adjusted so that thepiston is raised sufficiently high and sufficiently long to achieveeffective purging of the combustion chamber, this minimizes the pressuredeveloped by a specific charge, thus reducing the strength of theseismic wave generated. As the typical fuel combination of propane andoxygen does not burn cleanly, effective purging of the combustionchamber is essential to obtain efficient burning of fueland to avoidpremature explosions caused by burning residue setting off the fuelcharge as the combustion chamber is refueled. Thus there is an inherenttradeoff in the operation of a conventional gas gun between obtainingpeak charge pressure and effective combustion chamber purging.

Accordingly, another object of the present invention is to obtaineffective cleaning and purging of the cornbustion chamber without thedeficiency of reduced peak pressure.

In accordance with the stated objects, the gas gun of the presentinvention comprises a gas cushion valve which maintains exhaust pressurein the combustion chamber and then controls the release of the exhaustgases in such a manner as to avoid creation of secondary shock waves andyet obtain effective purging of the combustion chamber. The combustionchamber remains closed or relatively closed to the atmosphere during thecreation of peak combustion pressure.

For a more complete understanding of the present invention, referenceshould now be made to the following detailed description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a cross-section of a gas gun having a gas cushion valve inaccordance with the present invention;

FIG. 2 is a section view of the gas cushion valve;

FIG. 2a is an end view of the gas cushion valve of FIG. 2;

FIG. 3 is a second embodiment of a gas cushion valve;

FIG. 4 is a further embodiment of the present invention, utilizing aball check valve and controlled orifice to form a gas cushion exhaust;

FIG. 5 is a drawing of an embodiment of the invention which comprises avalve which is opened by an actuator in response to an external controlsignal to release the exhaust gases and pressure through a controlledorifice; and

FIG. 6 shows another embodiment operating upon the basic principles ofthe embodiment of FIG. 5.

Referring now to FIG. I, a typical gas gun is shown wherein, inaccordance with the invention, a gas cushion valve 1 is provided. Thegas gun comprises a heavy upper member 3 to which is movably attachedand held in place by retaining ring 5 a pan-shaped member 7, forming acombustion chamber 9 between the two members. In use, a suitable fuelmixture, such as oxygen and propane, is introduced under pressureintocombustion chamber 9 by means of injector 11. The member 3 is normallymounted onto a weighted object such as a truck, thus to provideadditional weight to hold member 7 against the earth. The pressurizedfuel mixture in combustion chamber 9 is ignited by a spark plug 13,thereby creating an explosion in combustion chamber 9. The force of theexplosion drives pan-shaped member 7 into harder contact with the earth,thereby creating an impulse seismic wave, and at the same time raisesupper member 3 and the truck to which it is attached vertically withrespect to member 7, thereby increasing the volume in combustion chamber9. Gas cushion valve 1 prevents member 3, under the additional weight ofthe truck, from immediately slamming down against member 7 at the end ofcombustion and introducing secondary shock waves into the earth, andthus interferring with the seismic recordings being made from theimpulse created by the explosion.

The preferred embodiment of the gas cushion valve of the invention isillustrated in FIG. 2. The valve comprises a cylinder containing apiston 17 slidably mounted on a rod 19. Connecting member 21 isthreadedly mounted at one end of cylinder 15 and also threadedly mountsinto the gas gun. At the other end of the chamber of cylinder 15, endmember 23- is threadedly mounted. A plurality of orifices 25 in endmember 23 and a plurality of ports 27 in cylinder 15 allow thecontrolled escape of air in the upper part of the cylinder chamber aspiston 17 is thrust upward by the explosion in the combustion chamber.The orifices 25 may be plugged or left open, depending upon fillpressure and other factors. Circular ports 29, which open into thecombustion chamber of the gas gun, extend nearly through connectingmember 21 at which point they interface with a circular groove 31. Anend view of the gas cushion valve of FIG. 2 is shown in FIG. 2a, to moreclearly depict the arrangement of the plurality of ports 29 and thegroove 31 in connecting member 21. Exhaust ports 33 in cylinder 15 allowthe controlled escape of exhaust gases and pressure from the combustionchamber. A plurality of circular grooves 35 in piston 17 provide a moreeffective initial seal (known as a labyrinth seal) between the piston 17and connecting member 21, thereby preventing the premature escape of thepressurized fuel mixture.

When the fuel mixture is introduced under pressure into the combustionchamber, this pressure is exerted against piston 17 by way of ports 29and groove 31. The weight of piston 17 is matched to the desired chargepressure, such that if an excessive charge is introduced, piston 17 willbe slightly raised and allow pressure to escape through exhaust ports33. This prevents an excessive charge from being exploded in the gun andbursting the pan-shaped member which is in contact with the ground, orotherwise damaging the gas gun. As excessive pressure is bled off,piston 17 drops back into place against connecting member 21, therebysealing off the combustion chamber from the atmosphere.

When the fuel mixture is exploded in the combustion chamber of the gasgun, force is exerted through groove 31 against piston 17. The piston isthrust upward, forcing air out of the piston chamber through orifices 25and ports 27. The size of orifices 25 and ports 27 is determined fromthe mass of piston 17 and desired explosive force such that the pistonwill not strike end member 23, or will do so with negligible force. Theslowly escaping air in the upper part of the piston chamber forms acushion and absorbs most of the energy transferred to piston 17 duringthe explosion.

Meanwhile, the exhaust gases and pressure are being released throughexhaust ports 33, which are very small as compared to the internalvolume of the gas gun. Exit of the exhaust gases is sufficientlydelayed, and controlled by the small area open to the atmosphere, thatvery near peak pressure is obtained from the exploded charge. Theexhaust gases continue to rush out of the combustion chamber under theirown pressure as well as the pressure created by the slow descent of theupper member of the gas gun and attached weight. The piston 17 reboundsagainst the outrushing gas stream, partly under force of gravity andpartly rebounded by the air cushion, and is again forced upward. By thetime the piston 17 settles into place against connecting member 21,sealing off the combustion chamber from the atmosphere, the momentum ofthe outrushing exhaust gases has created a partial vacuum in thecombustion chamber, thereby effectively purging it.

It should now be apparent that the preferred gas cushion valve achievesthe stated objects of the invention. The controlled release of theexhaust gases in the manner described above prevents the upper member ofthe gas gun from slamming down against the lower member and introducingsecondary shock waves. Near peak pressure is developed from a charge,and yet effective purging of the combustion chamber is obtained. In agas gun equipped with the gas cushion valve described above,approximately 30 percent more ener gy is introduced into the earth thanis introduced with a typical conventional gas gun that is loaded with anidentical fuel charge.

In one embodiment of the gas cushion valve of FIG. 2, designed for usein a gas gun having a 55-inch diameter pan, the piston 17 weighs 7.6pounds and has a 1.5- inch stroke. There are four exhaust ports 33 eachhaving an area of 0.1 10 square inches. There are two orifices 25 whichare l/32-inch in diameter. There are six ports 29 which are each 0.0625square inches in area. This gas cushion valve is designed to hold a fuelcharge fill pressure on the order of 7 l0 psi. A gas gun employing thegas cushion valve just described consistently generates impulse seismicwaves of approximately 30 percent more energy than the same gas gunemploying the conventional spring-loaded exhaust system, and loaded withan identical charge.

An alternate construction of the preferred type of gas cushion valve isshown in FIG. 3. A feature of this embodiment is that it is easy tomanufacture and assemble. Cylinder 37 contains piston 39 slidablymounted on a rod 41. Rod 41 is a large bolt which holds end member 43and connecting member 45 into place at the respective ends of cylinder37. The operation of this valve is identical to that already described.It will be noted that the circular grooves in the piston have beenreplaced by an extended ridge 47 which covers groove 49. Thisconstruction allows the escape of excessive charge pressure from thecombustion chamber but otherwise effectively seals off the combustionchamber from the atmosphere. The piston construction of FIG. 3 has beenfound to provide a better seal, more consistently maintaining thedesired fill pressure and opening to allow the exact amount of excessfill pressure to bleed off, than the labyrinth seal of FIG. 2.

In its most basic form the invention is illustrated in FIG. 4. Aconventional gas gun can be quite easily modified to comprise thisembodiment. The conventional spring-load exhaust system has been removedand the space left thereby is covered by a cover plate 51. Riser pipe 53extends through cover plate 51 and opens into the combustion chamber ofthe gas gun. At the upper end of riser pipe 53 is connected check valve55 and controlled orifice 57. If an excessive charge is introduced intothe combustion chamber, check valve 55 opens and bleeds off theexcessive pressure through exhaust port 59 and orifice 57. When the fuelmixture in the combustion chamber is ignited the force of the resultingexplosion closes check valve 55 by lifting ball 61 into its seat onexhaust port 59. This seals off the combustion chamber from theatmosphere and allows peak pressure and maximum energy to be developed.

The exhaust gases are trapped under pressurein the combustion chamberand prevent the upper member of the gas gun from rebounding against thelower member. As the gas gun cools, the pressure in the combustionchamber reduces and ball 61 will drop from its seat on exhaust port 59,thereby opening the combustion chamber to the atmosphere through exhaustport 59 and orifice 57. Orifice 57 can be adjusted to bleed off theexhaust gases at a rate which allows the truck and the upper member ofthe gas gun to descend slowly.

FIG. 5 shows another illustrative embodiment of the invention. Coverplate 51, riser pipe 53 and controlled orifice 57 are the same asdescribed immediately above. Valve assembly 65 is controlled by aremotely operated actuator 67. The actuator 67 may be operatedmechanically, electrically, hydraulically or pneumatically. Actuator 67is controlled by an external control signal which may be programmed tocause valve assembly 65 to open at any specified time after theexplosion has occurred. Thus, the gas cushion can be held to absorb therebound energy of the upper member of the gas gun, and then released ata controlled rate through orifice 57 to allow the upper member todescend gently. Maximum energy is obtained because the combustionchamber is completely closed to the atmosphere during the creation ofpeak pressure.

In the operation described immediately above, effective purging andcleaning of the combustion chamber will not be obtained. This may beaccomplished by the addition of a blower system 69. Blower system 69comprises a high volume, low pressure blower 71, two sliding disc gatevalves 73 and 75, actuator 77, riser pipe 79 and connecting pipe 81. Atthe end of the operating cycle as described above, actuator 77 isenergized to open valves 73 and 75. Clean air under pressure from blower71 flows through valve 73, through the combustion chamber, and outthrough valve 75. The actuator is then de-energized to cut off the flowof wash air. It should be apparent that any of the embodiments describedherein could be so equipped.

An alternative construction (not shown) of the embodiment of FIG. 5would consist of replacing controlled orifice 57 with a large diameterexhaust port. Valve assembly 65 would then preferably be a special typeof sliding disc gate construction having extremely large ports whichpermit high flow rates with minimum pressure drop. After the explosionin the combustion chamber, valve 65 remains closed until the seismicrecording has been completed. Then valve 65 is opened and the exhaustgases rush out through the large exhaust port as the truck and uppermember of the gas gun rapidly descend. The outrushing gases leave apartial vacuum in the combustion chamber, thereby effectively purgingit. I

The illustrative embodiment of FIG. 6 operates substantially similar tothe embodiment described immediately above, but eliminates the expenseand complication of employing an externally controlled actuator. Riserpipe 83 and passages 85 and 87 of exhaust valve 89 are large. Checkvalve 91 is spring loaded with just enough force to contain thefillpressure in the combustion chamber. An excessive charge will opencheck valve 91 and bleed it off to a safe pressure. When the fuelmixture is ignited the force of the explosion opens check valve 91,which begins to bleed off the exhaust pressure. Check valve 91 has verysmall ports which allow only a very small amount of pressure to bleedoff. As the pressure is slowly reduced in riser pipe 93, a pressuredifferential is created across flexible diaphragm 95 is exhaust valve89. The pressure differential causes passage to be opened to passage 87,allowing the exhaust gases to rush out and leave a partial vacuum in thecombustion chamber. This sudden outrush of the exhaust gases allows theupper member of the gas gun to drop suddenly against the pan-shapedmember, but check valve 91 can be adjusted so that the seismicrecordings will be completed before the pressure differential acrossdiaphragm is sufficient to open the exhaust passage.

The present invention having been described with particular reference tospecific embodiments thereof, certain modifications that fall within theinventive concept may now suggest themselves to those skilled in theart. The scope of the invention is defined by the following claims.

What is claimed is:

1. A gas gun for use in seismic exploration comprising:

a. a first member and a second member which form a combustion chambertherebetween, said second member being movable with respect to saidfirst member;

b. means for introducing a gaseous fuel mixture into said combustionchamber;

0. means for igniting said gaseous fuel mixture; and

d. a gas cushion valve coupled to said first member which comprises acylinder having a piston suspended therein, said cylinder having atleast one air escape port near the upper end thereof and at least oneexhaust port near the lower end thereof, and further having at least oneconnecting port which connects said lower end to said combustionchamber; whereby e. combustion pressure exerted against said pistonthrough said connecting port raises said piston, forcing an outrush ofair through said air escape port and allowing the escape-of exhaustpressure and gases through said exhaust port.

2. The gas gun of claim 1 wherein the weight of said piston is set tohold a predetermined fuel charge pressure in said combustion chamber.

3. The gas gun of claim 1 wherein said piston is slidably mounted on arod extending through said cylinder.

4. The gas gun of claim 1 further including upper and lower end memberscoupled to said cylinder to form a chamber, said upper and lower endmembers each having at least one port extending therethrough, andwherein said piston is slidably mounted on a rod extending through saidchamber.

5. A gas gun for use in seismic exploration, comprismg:

a. a first member and a second member which form a combustion chambertherebetween, said second member being movable with respect to saidfirst member;

b. means for introducing a gaseous fuel mixture into said combustionchamber;

c. means for igniting said gaseous fuel mixture, thereby raising saidfirst member with respect to said second member; and

. an exhaust system adapted to maintain pressure of combustion in saidcombustion chamber to prevent the first member thereof from reboundingagainst the second member thereof to introduce secondary seismic waves,and thence to release exhaust gases and purge said combustion chamber.

6. The gas gun of claim wherein said exhaust system comprises only avalve and an exhaust port, with said valve adapted to be externallyactivated to open and release exhaust gases through said exhaust portafter combustion.

7. The gas gun of claim 6 wherein said externally activated valve is asliding disc gate valve.

8. The gas gun of claim 5 wherein said exhaust system comprises only anexternally actuated valve and a controlled orifice.

9. The gas gun of claim 8 further including an externally actuatedblower system connected to said combustion chamber.

10. The gas gun of claim 5 wherein said exhaust system is comprised onlyof a check valve and a controlled orifice, with said check valve beingadapted to maintain a desired fuel charge pressure in said combustionchamber, to close under pressure of combustion, and to open uponsufficient cooling and reduced pressure to allow gradual escape ofexhaust gases through said controlled orifice.

1 1. A gas gun for use in seismic exploration, comprismg:

a. a first member and a second member which form a combustion chambertherebetween, said second member being movable with respect to saidfirst member;

b. means for introducing a gaseous fuel mixture into said combustionchamber;

c. means for igniting said gaseous fuel mixture; and

d. an exhaust system comprising a check valve and an exhaust valvehaving a diaphragm therein and further including a first connectingmeans interconnecting said combustion chamber to said check valve and toone side of said diaphragm and a second connecting means interconnectingsaid combustion chamber to the other side of said diaphragm and furtherconnecting to the atmosphere when a pressure differential develops,

across said diaphragm; whereby e. at a controlled time after ignition ofsaid fuel mixture exhaust pressure bleed off through said check valvecreates a pressure differential across said diaphragm, opening saidexhaust valve and releasing exhaust pressure and gases into theatmosphere.

12. A gas cushion valve adapted to control the release of exhaust gasesin a seismic gas gun to avoid introduction of secondary seismic waves,comprising a cylinder having upper and lower end members to thereby forma chamber, said lower end member having at least one port extendingtherethrough and said cylinder having at least one port extendingtherethrough near each end thereof, and a movable piston suspendedwithin said chamber.

13. The valve of claim 12 wherein said upper end member has at least oneport extending therethrqugh.

14. The valve of claim 13 further inc uding a circular groove in theupper portion of said lower end member and interfacing with said piston,and a plurality of selectively spaced ports in said lower end membercoupling said groove to said chamber.

15. The valve of claim 14 wherein said piston is slidably mounted on arod extending through said chamber.

16. The valve of claim 15 wherein said piston has a plurality of groovesin the end which interfaces with said lower end member to form a goodseal.

17. The valve of claim 15 wherein said piston has a circular ridge inthe end which interfaces with said lower end member, said ridge being atleast wide enough to cover said groove in said lower end member.

1. A gas gun for use in seismic exploration comprising: a. a firstmember and a second member which form a combustion chamber therebetween,said second member being movable with respect to said first member; b.means for introducing a gaseous fuel mixture into said combustionchamber; c. means for igniting said gaseous fuel mixture; and d. a gascushion valve coupled to said first member which comprises a cylinderhaving a piston suspended therein, said cylinder having at least one airescape port near the upper end thereof and at least one exhaust portnear the lower end thereof, and further having at least one connectingport which connects said lower end to said combustion chamber; wherebye. combustion pressure exerted against said piston through saidconnecting port raises said piston, forcing an outrush of air throughsaid air escape port and allowing the escape of exhaust pressure andgases through said exhaust port.
 1. A gas gun for use in seismicexploration comprising: a. a first member and a second member which forma combustion chamber therebetween, said second member being movable withrespect to said first member; b. means for introducing a gaseous fuelmixture into said combustion chamber; c. means for igniting said gaseousfuel mixture; and d. a gas cushion valve coupled to said first memberwhich comprises a cylinder having a piston suspended therein, saidcylinder having at least one air escape port near the upper end thereofand at least one exhaust port near the lower end thereof, and furtherhaving at least one connecting port which connects said lower end tosaid combustion chamber; whereby e. combustion pressure exerted againstsaid piston through said connecting port raises said piston, forcing anoutrush of air through said air escape port and allowing the escape ofexhaust pressure and gases through said exhaust port.
 2. The gas gun ofclaim 1 wherein the weight of said piston is set to hold a predetErminedfuel charge pressure in said combustion chamber.
 3. The gas gun of claim1 wherein said piston is slidably mounted on a rod extending throughsaid cylinder.
 4. The gas gun of claim 1 further including upper andlower end members coupled to said cylinder to form a chamber, said upperand lower end members each having at least one port extendingtherethrough, and wherein said piston is slidably mounted on a rodextending through said chamber.
 5. A gas gun for use in seismicexploration, comprising: a. a first member and a second member whichform a combustion chamber therebetween, said second member being movablewith respect to said first member; b. means for introducing a gaseousfuel mixture into said combustion chamber; c. means for igniting saidgaseous fuel mixture, thereby raising said first member with respect tosaid second member; and d. an exhaust system adapted to maintainpressure of combustion in said combustion chamber to prevent the firstmember thereof from rebounding against the second member thereof tointroduce secondary seismic waves, and thence to release exhaust gasesand purge said combustion chamber.
 6. The gas gun of claim 5 whereinsaid exhaust system comprises only a valve and an exhaust port, withsaid valve adapted to be externally activated to open and releaseexhaust gases through said exhaust port after combustion.
 7. The gas gunof claim 6 wherein said externally activated valve is a sliding discgate valve.
 8. The gas gun of claim 5 wherein said exhaust systemcomprises only an externally actuated valve and a controlled orifice. 9.The gas gun of claim 8 further including an externally actuated blowersystem connected to said combustion chamber.
 10. The gas gun of claim 5wherein said exhaust system is comprised only of a check valve and acontrolled orifice, with said check valve being adapted to maintain adesired fuel charge pressure in said combustion chamber, to close underpressure of combustion, and to open upon sufficient cooling and reducedpressure to allow gradual escape of exhaust gases through saidcontrolled orifice.
 11. A gas gun for use in seismic exploration,comprising: a. a first member and a second member which form acombustion chamber therebetween, said second member being movable withrespect to said first member; b. means for introducing a gaseous fuelmixture into said combustion chamber; c. means for igniting said gaseousfuel mixture; and d. an exhaust system comprising a check valve and anexhaust valve having a diaphragm therein and further including a firstconnecting means interconnecting said combustion chamber to said checkvalve and to one side of said diaphragm and a second connecting meansinterconnecting said combustion chamber to the other side of saiddiaphragm and further connecting to the atmosphere when a pressuredifferential develops across said diaphragm; whereby e. at a controlledtime after ignition of said fuel mixture exhaust pressure bleed offthrough said check valve creates a pressure differential across saiddiaphragm, opening said exhaust valve and releasing exhaust pressure andgases into the atmosphere.
 12. A gas cushion valve adapted to controlthe release of exhaust gases in a seismic gas gun to avoid introductionof secondary seismic waves, comprising a cylinder having upper and lowerend members to thereby form a chamber, said lower end member having atleast one port extending therethrough and said cylinder having at leastone port extending therethrough near each end thereof, and a movablepiston suspended within said chamber.
 13. The valve of claim 12 whereinsaid upper end member has at least one port extending therethrough. 14.The valve of claim 13 further including a circular groove in the upperportion of said lower end member and interfacing with said piston, and aplurality of selectively spaced ports in said lower end member couplingsaid groove to said chamber.
 15. The valve of claIm 14 wherein saidpiston is slidably mounted on a rod extending through said chamber. 16.The valve of claim 15 wherein said piston has a plurality of grooves inthe end which interfaces with said lower end member to form a good seal.